The concept of all-wheel steering is well known. Dozens of all-wheel steering systems are disclosed in U.S. and foreign patents. Some known all-wheel steering systems are purely mechanical, meaning that the rear wheel angle is determined by gears and gear ratios manually selected by the vehicle operator with the shift of a lever. Other all-wheel steering systems provide rear wheel steering independent of the front wheel angle where the operator steers the rear wheels independently of the front wheels, the rear wheels being steered with a separate steering wheel, joy stick or potentiometer.
Automatic methods of all-wheel steering are also known. In these systems, a microcomputer will control the rear wheel angle in accordance with various physical relationships that are functions of various parameters. For example, it is known to control the rear wheel angle in accordance with the front wheel-to-rear wheel steering angle ratio. It is also known to control the rear wheel angle indirectly by controlling the rear wheel speed so that the desired rear wheel speed is nearly equal to the actual front wheel speed. Other systems control the rear wheel angle after solving a series of complex equations that are functions of the steering angle, vehicle speed and other parameters. Finally, in some all-wheel steering systems, the rear wheel angle is a function of the speed of rotation of the steering wheel.
Our concept of all-wheel steering has been applied to larger vehicles such as trucks and aircraft rescue and fire fighting equipment (ARFF). Larger vehicles generally have high centers of gravity which makes them prone to roll-overs if large lateral g-forces are experienced. All-wheel steering allows larger vehicles to maneuver safely at high speeds because all-wheel steering systems can reduce the lateral g-forces imposed on the vehicle during sharp turns and lane changes at high speeds. Further, many trucks, including fire fighting trucks, must be able to maneuver into small spaces in short periods of time. All-wheel steering provides great benefits in this regard because it provides a smaller minimum turning radius.
The prior art teaches all-wheel steering systems with four principal steering modes: front wheel only, crab, coordinated and independent rear wheel. In front wheel only steering, the steering of the vehicle is dependent upon the front wheel position only and the rear wheels maintain a straight-ahead position, or 0.degree. steering angle. Front wheel only steering is appropriate at both low and high speeds.
In crab steering, the front wheels and rear wheels are steered in the same direction. Crab steering improves the vehicle's stability at high speeds allowing the vehicle to change lanes and avoid obstacles with less vehicle yaw and improved traction. Crab steering lowers the lateral g-forces by increasing the turning radius for a given front wheel angle thereby lowering the chances of roll-overs for vehicles with relatively high centers of gravity, such as aircraft, rescue and fire fighting vehicles (ARFF). Because the rear of the vehicle is also steered to the side by the rear wheels, the increased turning radius is not detrimental to obstacle avoidance. Crab steering is also beneficial to passenger cars because it allows the driver to change lanes or avoid obstacles quickly and safely. Crab steering is effective at moderate to high speeds.
In coordinated steering, the front wheels and rear wheels are turned in generally opposite directions. Coordinated steering is safely employed for low speed maneuvers only. Coordinated steering improves the vehicle's maneuverability by reducing the minimum turning radius. This increased maneuverability is especially beneficial to vehicles with long wheel bases such as heavy duty trucks and ARFF equipment, but is also greatly appreciated by the drivers of passenger cars. The reduced minimum turning radius makes it easier for the drivers of passenger cars as well as trucks to maneuver and park in congested city traffic.
Further, in independent rear wheel steering, the rear wheels are controlled independently of the front wheels. Rear wheel steering requires an additional and separate steering mechanism for the rear wheels which is separate and apart from the conventional steering wheel used to control the front wheels.
The present invention makes several contributions to the art of all-wheel steering. First, the present invention provides two improved means for calculating or selecting the desired rear wheel angle based upon the front wheel angle. A first means of calculating the desired rear wheel angle employs a relational curve to be used at moderate to high speeds and features a programmable "dead band" that allows a certain amount of front wheel movement during which the rear wheels will remain in the straight-ahead or 0.degree. position. A second improved means for calculating the desired rear wheel angle employs a relational curve that may be used for straight-ahead, coordinated and crab modes of steering. The operator or programmer may select from a variety of relational curves or polynomial equations in order to tailor the front to rear relational curves to the vehicle. Further, the driver may select from a variety of relational curves so that different relational curves may be provided for different driving conditions.
Further, the present invention alleviates the steering problem known as off-tracking that is most commonly associated with trucks and trailers or tractor-trailer rigs. Off-tracking occurs when the driver turns causing the side of the trailer or vehicle to move laterally in the direction of the turn. The amount of off-tracking is proportional to the turn and off-tracking occurs to a certain extent for all turns greater than 0.degree.. If the driver doesn't take the turn at an angle that is wide enough, the trailer or vehicle can engage the curb or an object located on a street corner, such as a signal light. All wheel steering can be used with trailers to allow tractor-trailer rigs to make sharp turns in the coordinated mode. In the coordinated mode, the trailer wheels and particularly the rear wheels of the trailer will steer in a direction opposite of the turning direction or the direction of the front wheels of the tractor thereby steering the trailer around obstacles located on a street corner. In effect, the trailer more accurately tracks the path of the tractor thereby alleviating the off-tracking problem.
In one preferred embodiment, a delay in turning the rear wheels of the trailer enhances the tracking of the rig through a turn because the delay enables the trailer to more accurately follow the path of the tractor without the tractor travelling outside of the truck's path at the start of the turn. Crab steering enhances the obstacle avoidance and lane-changing ability of tractor-trailer rigs and is useful when backing up tractor-trailer rigs because it makes them behave like straight trucks.
Finally, the present invention allows the driver to manually select from front wheel only, crab, coordinated and independent rear wheel steering modes as well as a variety of all-wheel steering modes.